Improved spatial resolution in flat-panel imaging systems

Results of an investigation into the limiting spatial resolution of a flat-panel amorphous silicon (a-Si:H) X-ray imaging system are reported. The system was comprised of a 127 μm pixel pitch a-Si:H array used in conjunction with an overlying Gd₂O₂S:Tb (GOS) phosphor screen. The pre-sampled modulation transfer function (psMTF) of the system was measured at diagnostic X-ray energies and compared to the value predicted from a knowledge of the spatial resolution of the individual system components. A reproducible drop in the measured psMTF is seen at low spatial frequencies. Measurements of the magnitude of X-ray backscatter from the array substrate, along with the results of a theoretical model for K-fluorescence X-ray scatter, indicate that a significant fraction of this low-frequency drop is due to K-fluorescence from heavy elements in the glass substrate of the array. This K-fluorescence may be excited directly by primary X-rays that penetrate the overlying phosphor and interact in the glass, or by gadolinium K-fluorescence X-rays that escape from the phosphor into the glass. The measurements indicate that the spatial resolution of such an X-ray imaging system may be improved by the use of a substrate containing as low a concentration of heavy elements as possible.